Highly alloyed metals, i.e., metal alloys containing substantial amounts of additional metal elements other than their base metal elements, exhibit many desirable properties including superior strength and corrosion resistance.
FIG. 1 illustrates isothermal time-temperature-transformation (TTT) diagrams for such an alloy, in particular an AISI-2205 duplex stainless steel. When such an alloy is maintained at conditions of time and temperature within the envelop of its particular TTT curve the individual elements forming the alloy tend to segregate from one another, with some of these elements combining with one another to form discrete intermetallic phases. Thus, each curve in FIG. 1 shows how much (i.e., 1%, 3%, 5%, and 10%) of the deleterious intermetallic sigma phase will form in this alloy when held at a particular temperature for a particular time. For example, FIG. 1 shows that holding this alloy at a temperature of about 860° C. for a soak period of about 2 minutes leads to the precipitation of 1% sigma phase in the alloy. Likewise, holding this alloy at this same temperature for about 7 minutes causes the formation of 5% sigma phase.
The extrapolation of the upper portion of TTT-curves to very long times yields an upper critical temperature above which intermetallic phases are thermodynamically not stable. The extrapolation of the lower portion of TTT-curves to very long times leads to a lower critical temperature below which intermetallic phases do not form for kinetic reasons. The temperature range defined by the upper and lower critical temperatures is called the critical temperature range for intermetallic phase formation. If the alloy is held at a temperature above the critical range, all of the elements in the alloy including those already present in intermetallic phases, tend to redistribute themselves into a uniform solid solution. Meanwhile, once the alloy is at a temperature below the critical range, the elements in the alloy are completely immobile with respect to one another no matter how long the alloy is held at that temperature.
The presence of these intermetallic phases in more than insignificant amount is detrimental to the properties of the alloy. As a result, it is standard practice in industry to subject hot forgings made from highly alloyed metals to a post-forge solution annealing treatment. Solution annealing involves heating an alloy up to, and maintaining the alloy at, an elevated temperature above the upper critical temperature for intermetallic phase formation. As a result, the atomic elements which have formed the intermetallic phases go back into solid solution with all the other elements of the alloy. Thereafter, the alloy is rapidly quenched through its critical temperature range so that formation of these intermetallic phases is avoided or at least minimized.
This is illustrated in FIG. 2, which shows continuous cooling curves, or “CCT curves,” for this alloy. For example, FIG. 2 shows that, if such an alloy is cooled from 950° C. to below 600° C. according to the cooling regime represented by the solid line in this figure, it will develop about 1% deleterious sigma phase. On the other hand, if the alloy is cooled by the cooling regimes represented by the other lines in this figure, it will develop about 3%, 5% or even 10% of this deleterious sigma phase depending on which cooling rate is followed.
Because of the time and complexity involved, post forge solution annealing is expensive. In addition, it may also lead to various technical and commercial problems such as surface oxidation, lower mechanical properties due to grain growth, added production time and cost and negative environmental impact including consumption of energy and cooling water. Accordingly, it would be desirable to eliminate post forge solution annealing altogether, if possible.